How does the National Grid carry electricity efficiently across the country?
The structure of the National Grid and why electricity is transmitted at high voltage to reduce energy loss.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on the National Grid, covering its structure and why electricity is transmitted at high voltage and low current to reduce energy loss in the cables.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
WJEC Double Award Unit 3 wants you to describe the structure of the National Grid and explain why electricity is transmitted at high voltage to reduce energy loss.
What the National Grid is
It links many power stations together so electricity can be sent wherever it is needed, and so that demand can be met across the whole country.
Why high voltage is used
If the same power were sent at a low voltage, the current would be very large, and a great deal of energy would be lost heating up the cables.
The role of transformers
The voltage is changed by transformers along the way:
- A step-up transformer raises the voltage (and lowers the current) as the electricity leaves the power station, for efficient transmission.
- The electricity is carried across the country at this high voltage through the pylon cables.
- A step-down transformer lowers the voltage again to a safe value (about 230 V) before it reaches homes and businesses.
Balancing efficiency and safety
High voltage is efficient for transmission but is dangerous to use directly, which is why it must be stepped down before reaching homes. The grid therefore balances efficiency (high voltage for the long-distance cables) with safety (low voltage for use). This is why both step-up and step-down transformers are essential parts of the system.
Why the grid is useful
Because power stations are linked, electricity can be routed to areas of high demand, and several sources (including renewables) can feed in. The grid also allows power stations to be built away from towns, near fuel or cooling water, while still supplying cities. These advantages depend on being able to transmit power efficiently over long distances, which is what high-voltage transmission makes possible.
Overhead and underground cables
Most transmission uses overhead cables held up by pylons, because they are cheaper to install and easy to repair, and the surrounding air helps to cool and insulate them. Underground cables are sometimes used in towns or areas of natural beauty: they are hidden and not affected by storms, but they are far more expensive and harder to repair. Exam questions often ask you to weigh up these options, so it helps to know that overhead lines are chosen mainly for cost while underground lines are chosen mainly to protect the landscape.
Matching supply to demand
The demand for electricity changes through the day, rising in the morning and evening and falling at night. The National Grid must match the supply to the demand at all times, switching power stations on or off and drawing on different sources as needed. Linking many power stations together makes this possible, because spare capacity in one area can cover a shortfall in another. This is another reason a national network is used rather than each town having its own isolated supply.
Try this
Q1. Why is the current kept low during transmission? [1 mark]
- Cue. To reduce the energy lost as heat in the cables.
Q2. What does a step-down transformer do in the grid? [1 mark]
- Cue. Lowers the voltage to a safe level for homes and businesses.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain why electricity is transmitted across the National Grid at very high voltage.Show worked answer →
A Unit 3 explain question worth 4 marks. Reward: transmitting at high voltage means the current is low for the same power (since power = current x voltage) (1); a lower current means less energy is lost as heat in the cables (1), because the heating depends on the current (2); so high-voltage transmission is more efficient (1). Markers credit the low current, the reduced heat loss in the cables and the efficiency. A common error is to say high voltage itself causes less heating, rather than the low current it allows.
WJEC style3 marksDescribe the role of step-up and step-down transformers in the National Grid.Show worked answer →
A Unit 3 describe question. Reward: a step-up transformer increases the voltage (and lowers the current) for efficient transmission across the grid (1); the electricity is carried at high voltage through the power lines (1); a step-down transformer then decreases the voltage to a safe level for homes and businesses (1). Markers credit step-up for transmission, the high-voltage transmission, and step-down for safe use. A common error is to swap the two transformers.
Related dot points
- Electromagnetic induction in a generator, how a transformer changes voltage, and the difference between step-up and step-down transformers.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on generators and transformers, covering electromagnetic induction in a generator, how a transformer changes voltage, and step-up and step-down transformers.
- Renewable and non-renewable energy resources, how power stations generate electricity, and the advantages and disadvantages of different resources.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on energy resources, covering renewable and non-renewable resources, how power stations generate electricity, and the advantages and disadvantages of each resource.
- Electric current, potential difference and resistance, the equation V = IR, and the current-voltage graphs for a resistor, a filament lamp and a diode.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on electricity, covering current, potential difference and resistance, the equation V = IR, and the current-voltage characteristics of a resistor, filament lamp and diode.
- Mains alternating current, the live, neutral and earth wires in a plug, the dangers of electricity, and how fuses, circuit breakers and earthing provide safety, with the power equation.
A focused answer to the WJEC GCSE Science Double Award Unit 3 topic on domestic electricity, covering mains alternating current, the three wires in a plug, electrical dangers, fuses, circuit breakers and earthing, and the power equation P = IV.